A stellar polymer is a polymer comprising a central portion and linear polymer arms radially extending therefrom and is known to have various properties differentiating itself from linear polymers. The methods of producing stellar polymers are roughly classified into two groups. In one group, polymer arms are grown from a compound constituting the center and, in the other, polymer molecules to constitute arms are first produced and then joined together to form a stellar form. As a method usable for joining the arms, there can be mentioned the method comprising reacting the arms with a compound having a plurality of functional groups capable of reacting with the terminal functional groups of the arms and the method comprising adding a compound having a plurality of polymerizable groups following polymerization of the arms.
The polymers capable of constituting such stellar polymers may be homopolymers or copolymers, including various species such as polystyrene, poly(meth)acrylates, polydienes, polyethers, polyesters and polysiloxanes, among others. For obtaining controlled stellar structures, it is necessary that the polymerization be controlled irrespective of which method is employed, so that anionic polymerization, living cationic polymerization or polycondensation is employed in most instances.
Apart from those polymers obtained by ionic polymerization or polycondensation mentioned above by way of example, vinyl polymers obtained by radical polymerization and having a stellar structure have virtually not been put to practical use. In particular, the method of achieving chain extension or constructing a stellar structure by joining polymerization growing termini has failed. Among vinyl polymers, (meth)acrylic polymers have some characteristics, such as high weathering resistance and transparency, that are not possessed by the above-mentioned polyether polymers, hydrocarbon polymers or polyester polymers. Thus, (meth)acrylic polymers having an alkenyl group or crosslinking silyl group on a side chain have been used in high weathering resistance paint compositions and so forth. However, it is not easy to control the polymerization for acrylic polymers owing to side reactions, and therefore it is very difficult to realize chain extension or construct a stellar structure.
Accordingly, the present invention has for its object to provide a method of producing a polymer comprising a radical polymerizable vinyl monomer and having extended chains or a stellar structure on polymerization as well as the polymer, further, to a composition in which said polymer is used.